Batch mixers exist for many industries, and generally involve combining two or more constituent elements into a container and mixing them until a homogeneous mixture exists. In mixing cement slurries for the oil drilling and production industry, a cement slurry must be mixed from its solid and liquid constituent elements in very precise proportions in order to obtain desired resultant properties in the mixture.
When only a small volume of a cement slurry is required to be pumped in a well, a batch mixing process is often used. The batch mixing process is very simple to control since it consists of mixing a predetermined volume of solids with a predetermined volume of liquid.
However, when larger volumes of a cement slurry are required, the slurry must be mixed continuously as it is pumped downhole into a well. Generally this is accomplished by continuously adding and mixing the solid and liquid constituent elements of the cement slurry into the mix tub, while simultaneously pumping the mixed slurry out of the mix tub. A problem with this process is that it requires precise and continuous control of the “addition rates” or the rates at which the solid and liquid constituents are added into the mix tub.
Although the addition rates of the liquid constituents are easily measured and controlled, measuring and controlling the addition rates of the solid constituents is much more difficult due to the inconsistency of solids flow, which is often due in part to the degree of fluidization of the solids mixture, variations in the packing or compactness of the solid mixture, the moisture content of the solid mixture, and/or other variations in the environment of the solid mixture or other variations in the solid mixture itself. In fact, due to these difficulties associated with the solid addition rate, a common mixing method involves measuring the density of the resultant slurry mixture within the mix tub, rather than directly measuring the rate of addition of the constituent elements.
Using such a method, the rates of addition of the solid and liquid constituents are continuously modified to maintain the mixture density at a desired target value. Such a process requires an experienced, skilled operator in order to obtain the desired proportions of the solid and liquid constituents in the slurry mixture, and is complicated by the relatively large lag time which exists between a change in the addition rate of the constituents and the corresponding change in the density measurement of the resultant slurry mixture.
As a result of the difficulty in continuously controlling the addition rate of the solid constituents, it is considerably more difficult to consistently mix to the desired accuracy with a continuous mixing process than it is with a batch mixing process. Accordingly, a need exists for a process that combines the simplicity and precision of a batch mixing process with the unlimited volume output of a continuous mixing process.